The present invention relates to an apparatus used in communications among data processing devices, for example personal computers. More specifically, the present invention is directed to a device useful for increasing the range of address recognition capabilities in a local area network (LAN) controller.
In communication system which operate by way of having one or more destination stations respond to addressed packets or frames of information, the operating speed of the system would be significantly reduced if data transmission would have to wait while each station decodes the address to determine if it is to receive or ignore the data. Accordingly, the destination stations in such a system preferably store the beginning of the information temporarily, during the analysis of the accompanying address information. This temporary storage, or "buffering" of the data, is especially useful in the implementation of a local area network based upon the Token Ring Access Method and Physical Layer Specification, ANSI/IEEE Std. 802.5-1985, ISO Draft Proposal 8802/5, approved Dec. 13, 1985, such as local area network hereinafter referred to as a "token ring LAN". The token ring LAN has a number of stations, such as personal computers, interconnected via LAN adapter circuitry into a serial ring, so that data is transmitted from one station to another on the ring through all of the intervening stations. A description of the construction and operation of such adapter circuitry is given in U.S. Pat. No. 4,571,675, issued Feb. 18, 1986, and assigned to Texas Instruments Incorporated. An address field is provided within the data "frame", so that the LAN adapters of such intervening stations will ignore data which is intended for another station on the ring. However, in order to maximize the communications rate, each LAN adapter on the token ring LAN decodes the address field in parallel with the storage of data following the address field. In the event that a rather small address field (capable of addressing a limited number of stations on a ring) is to be used, a FIFO or `first-in-first-out` buffer can be used to store the first data in the frame after the address field.
A larger address field, comprising multiple data words, may require that the words at the start of a frame may be stored in a larger memory within the LAN adapter, such as random access memory. If, as a result of comparing the address field of the frame, a LAN adapter determines that the frame is not one which it should copy (i.e., retain the memory), the memory space used to store the frame data may be recovered merely by writing data from the next frame into those locations in memory which stored the prior frame data. An integrated circuit, such as the TMS 38020 protocol handler integrated circuit manufactured and sold by Texas Instruments Incorporated, is generally provided for each LAN adapter on the token ring LAN to not only perform the address field comparison function described above, but also to control the necessary operations to allow utilization of the frame data or to recover the memory space used in the storage of the frame data, depending upon the result of the address field comparison function. U.S. Pat. No. 4,674,086, issued June 16, 1987, and assigned to Texas Instruments Incorporated contains a description of the construction and operation of a protocol handler integrated circuit such as the above-referenced TMS 38020.
However, a token ring LAN may be desired which provides for an addressing scheme beyond that utilized in a single token ring LAN system in which the addressing field specifies a certain station on the ring. For example, since the number of stations on a single ring may be limited to a finite number (e.g., 256 stations), stations may be set up which are resident on more than one ring (such stations generally termed "bridges"), thereby providing a path for communication from a station on a first ring to a station on a second ring. An example of an addressing scheme in such an arrangement is the source routing bridging scheme, where the station which is the source of the data will send out a series of address fields specifying the ring and address of the destination station, as well as the rings and bridges between the source and destination through which the data is to be routed. In such an arrangement, depending upon the particular configuration of the rings and stations, the address fields and accompanuing comparison schemes will necessarily be more complex than those which are incorporated into the protocol handler (or functionally equivalent) circuit. Since the specific configuration of the rings and bridges within the LAN may be customized for a particular application, the costs of designing and producing a new protocol handler circuit incorporating the new address field comparison logic are often prohibitive; this is especially true where not all of the stations will serve as bridges, and thus will not need the additional comparison logic. Accordingly, logic hardware external to the protocol handler circuit is useful in providing such customized comparison functions for LAN adapters associated with such bridge stations, leaving the original comparison functions in place and operable for non-bridge stations, and for bridge stations where the bridge station is itself the final destination of the data frame.
The external logic utilized for the extended address field comparison is likely to be neither synchronous nor consistent with the address field comparison which is performed internal to the protocol handler circuit. Accordingly, it is necessary to provide an interface between the protocol handler circuit and such external logic.
It is therefore an object of this invention to provide a protocol handler having an interface to external address field comparison logic in a token ring LAN adapter.
It is a further object of this invention to provide such an interface which, during the external comparison function, allows the storage of the data frame in random access memroy in a recoverable fashion.
It is a further object of this invention to provide such a protocol handler which is independent of the type of address field under comparison.
Other objects will be apparent to those of ordinary skill in the art having reference to this description and the accompanying drawings.